Method and apparatus for repair of reflective photomasks

ABSTRACT

A method of selectively ablating an undesired material from a substrate includes providing a substrate with two regions; providing laser pulses; tuning a wavelength of the laser pulses to match a desired wavelength characteristic of a material and directing the tuned laser pulses onto the substrate; and controlling a pulse duration, wavelength, or both, of the laser pulses to ablate the undesired material without damaging the substrate or any adjacent material. In another embodiment, an apparatus for repairing a defect on a reflective photomask includes a femtosecond pulse width laser; a harmonic conversion cell; a filter for passing a selected EUV harmonic of the laser light; a lens arrangement configured to direct the selected EUV harmonic of the laser light onto the photomask; and a control unit connected to the laser to control an ablation of the defect on the reflective photomask.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation under 35 U.S.C. § 120 of application10/660,477 filed on Sep. 12, 2003, now U.S. Pat. No. 7,170,030 theentire contents of which are incorporated herein by reference.

BACKGROUND

This invention generally relates to an apparatus and method for removalof materials using laser light. More particularly, it relates tomatching the wavelength of an ultrashort laser pulse to the absorptioncharacteristics of the material being removed, in order to enhance theremoval or ablation of one material, while leaving adjacent materialswith different absorption properties undisturbed. Even moreparticularly, it relates to the use of ultrashort duration laser pulsesto repair opaque defects on reflective photomasks. More particularly itrelates to the removal of defects on reflective photomasks by matchingthe wavelengths of the ablative laser light to the peak of thereflectance of the underlying photomask substrate.

Photomasks are extensively used in the fabrication of integratedcircuits on semiconductor wafers. While standard photomasks include apatterned absorbing or opaque film on a transparent substrate, areflective mask includes a reflective substrate coated with anabsorbing, opaque, or lower reflectivity patterned material. A metal,such as chromium, having a thickness on the order of about 1000 Å, isoften used as the opaque or absorbing film. Other examples of absorbingor non-reflective materials include TaN, TaSiN and TaBN.

Fabrication of reflective photomasks first involves production of areflective substrate. In the visible range of wavelengths, the substratemay be a piece of glass or other stable material. The substrate is thencoated with an alternating series of dielectric films to form areflective stack. Materials, whose optical thickness corresponds toone-quarter of the wavelength of the incident light, are deposited inlayers onto the substrate to form a multilayer coating. The layeredmaterials have alternating relatively high and low dielectric constantssuch that the reflected wavefronts from each interface constructivelyinterfere in the backward or reflected direction. Reflectivities canapproach 100% in the deep ultraviolet (UV), UV, visible, and nearinfrared (IR) regions of the optical spectrum. In some cases, thereflective substrate may be a metal which is highly reflective at aparticular wavelength of light.

This construct may work effectively in a vacuum and in extremeultraviolet (“EUV”) regions of the optical spectrum as well. In thosecases, multilayer films can be made with alternating layers of absorbingand non-absorbing films, where thin layers of absorbing films arepositioned at the node of the standing wave field within the multilayerstack. A phase shift of 180 degrees on reflection from each filmproduces constructive interference in the backward or reflectingdirection. Reflectivities can approach 70-80% in the EUV region of theoptical spectrum.

Upon production of the reflective substrate, an absorbing ornon-reflective material, as mentioned above, is then deposited onto thesubstrate, followed by an electron beam or photon beam sensitive organicresist. The resist is exposed with a high resolution technique, such asan electron beam, and developed to form the desired pattern in theresist. This pattern is then transferred into the absorber by etching,leaving both opaque/non-reflective and reflective regions on the mask.

The above-described conventional photomask manufacturing process usuallyresults in at least some imperfections, and defects are thereforefrequently encountered during inspection of the photomasks. In advancedmask production, the defect rate per mask approaches 100%, which isunacceptable for cost-effective manufacturing. Defects are categorizedas either “clear defects” or “opaque defects”. Clear defects are regionsdesigned to have the absorber present, but which actually do not haveabsorber. Opaque defects are regions designed to be clear of absorber,but which actually do have absorber. FIG. 1 illustrates typical defectsfound on photomasks, such as a bridge defect, a bump or extensiondefect, or an isolated defect.

When a defect is a bridge defect connected to an adjacent absorber line,as in FIG. 1, conventional laser ablation may damage that adjacent line,undesirably removing some wanted absorber from the line. In addition,because a relatively high amount of thermal energy can be transmittedwith the laser beam, the laser ablation step not only melts andvaporizes the unwanted metal defect region, it may also damage andremove a layer of substrate underlying and adjacent the opaque defect,producing roughness in the substrate. This damaged region of the quartzor glass substrate is also responsible for reduced reflectivity andaltered phase of reflected light.

As an alternative to laser ablation, conventional focused ion beam (FIB)techniques offer a very controlled process for sputtering a small regionof unwanted material. The ion beam can, in principle, be focused to amuch smaller size than the laser beam. In addition, the ion beamphysically sputters material, transmitting very little thermal energy tothe mask. However there are a number of problems that limit the use ofFIB for mask repair.

If the substrate is insulating, the ion beam rapidly charges thesurface, and both the ability to aim subsequent ions and to use the ionbeam to image the results is degraded. Second, while an opaque defect isbeing removed, substrate at the edge of the defect is attacked at thesame rate, and the result is a “river bed” or trench of damagedsubstrate around the defect. The substrate in this region has alteredreflectance and phase. Third, the focused ion beam species is typicallygallium, and gallium has been found implanted into the substrate whenthe opaque defect is removed, causing reflectance losses. Fourth, thesputtering of material by the ion beam leads to ejection of material inall directions, and some of this ejected material may come to rest onadjacent edges.

A more general problem involves the ablation of a specific material,patterned or otherwise, that resides in a matrix of other materials,without damaging the desirable materials surrounding the specificmaterial to be removed. What is needed then is an ablation process andapparatus or device that could be tuned to specific absorptionproperties of materials, thereby permitting the removal of one material,while leaving other materials in the matrix undisturbed.

SUMMARY

In this disclosure, we describe a method and apparatus that solvesconventional problems which, in various aspects of embodiments of theinvention, direct a relatively short pulse of light tuned to thewavelength of highest reflectivity of the substrate, and which ablatesthe absorbing defect in a non-thermal process which removes the defectwithout damaging the underlying reflective substrate.

In one aspect of the invention, defects in the patterned absorber atop areflective substrate can be removed by directing an intense ultrashortduration laser pulse just above the defect in order to ablatively removeit, without damaging the underlying reflective mask. Another aspect ofthe invention provides a general method of performing an ablativeprocess that leads to the specific removal of material. Another aspectof this invention provides a method of removing specific materialimbedded in a matrix of different materials, without any repair induceddamage or substrate pitting. In various aspects of embodiments of theinvention, a mask may be repaired without degradation of reflectance inthe defect region or in the region adjacent thereto.

In one embodiment of the invention, a method of selectively ablatingmaterial includes providing a substrate underlying regions of at leasttwo different materials; providing laser pulses having tunablewavelengths; tuning the laser pulses to a wavelength corresponding to adesired material characteristic of at least one of the at least twomaterials; and shining the tuned laser pulses onto the regions of atleast two materials until a portion of one of the at least two materialsis ablated from the substrate. Tuning may include tuning the wavelengthof the laser pulses to approximate a peak absorption wavelength or apeak reflection wavelength of one of the at least two materials. Thelaser pulses may be tuned to remove the portion of the at least twomaterials without damaging the underlying substrate. The pulse width ofthe laser pulses may be controlled, along with the wavelength, or both,to control an amount of energy applied to the materials or substrate.

In another embodiment, a method of producing an essentially defect-freephotomask for semiconductor applications includes providing a substrateincluding a surface having an absorbing layer patterned thereon as amask to yield a circuit when transferred to a resist coated wafer;inspecting the mask and detecting a defect on the mask in a defectregion; and directing energy on said defect region and removing asubstantial portion of said defect. The mask may include a reflectivesubstrate, such as a reflective metal, a reflective substrate having astack of dielectric layers arranged to be reflective at a specifiedwavelength of light, reflective substrate having a single layer ofmaterial reflective in a visible region of light, or a reflectivesubstrate having multiple layers of material reflective in a visibleregion of light.

In another embodiment of the invention, a method of repairing a defecton a mask includes providing a reflective substrate comprising the maskwhich includes a first region and a second region, wherein a lightabsorbing first material covers the first region and the second regionis free of the light absorbing material; inspecting the mask anddetecting an essentially opaque defect on the mask in a defect region;shining a plurality of laser pulses on the defect region to ablate thedefect after selecting a laser pulse duration; and removing the defectwithout damaging the reflective substrate underlying the defect. In thisaspect, the absorbing region may have an edge with an edge placementtolerance, wherein removing the defect leaves the edge of the absorbingregion within the edge placement tolerance. The placement tolerance maybe 10% or less, and the defect may be removed without splattering anydefect material on the substrate, or without pitting the substrate.

In another embodiment of the invention, a method of removing materialfrom a reflective substrate includes providing a reflective substratehaving a region with a non-reflective or absorbing material thereon; andshining a plurality of laser pulses on said non-reflective or absorbingregion to remove said material without damaging said reflectivesubstrate underlying said material. The reflective substrate may be amask, and the material may be a defect in a mask, which may includechrome or molybdenum or similar materials, for example.

Another embodiment of the invention is an apparatus for repairing adefect on a reflective photomask including a laser capable of providingfemtosecond pulse width laser light; a harmonic conversion celloptically coupled to the laser; a filter for blocking a fundamentalwavelength and passing a selected harmonic of the laser light; anobjective lens arrangement configured to provide the selected harmonicof the laser light onto the reflective photomask; and a control unitoperatively connected to the laser to control an ablation of the defecton the reflective photomask. The selected harmonic may be at an EUVwavelength, or other harmonic wavelength of interest.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the invention will be morereadily understood upon consideration of the following detaileddescription of the invention, taken in conjunction with the accompanyingdrawings in which:

FIG. 1 illustrates examples of typical defects including a typical“bridge” defect, a typical line/bump defect and a typical isolatedabsorber defect on an otherwise reflective photomask substrate field;

FIG.2A represents the ablative removal of a first material embedded in adifferent second material using a short pulse of laser light focusedjust above the surface of the materials;

FIG. 2B represents the ablative removal of a first material embedded ina different second material using a short pulse of laser light focuseddirectly on or near the surface of the materials;

FIG. 3A represents the irradiation of a reflective photomask substrate320 with an absorbing material 330 atop substrate 320, with the laserlight focused just above the surface of the materials;

FIG. 3B represents the irradiation of a reflective photomask substrate320 with an absorbing material 330 atop substrate 320, with the laserlight focused directly on or near the surface of the materials;

FIG. 4 displays, schematically, a side view of a photomask including amultilayered reflective substrate having an absorbing or non-reflectivepatterned material on a surface of the multilayered reflectivesubstrate;

FIG. 5A displays a technique of focusing the incident light above anabsorbing or non-reflective defect to ablatively remove it whileminimizing any damage to the substrate;

FIG. 5B displays a technique of focusing the incident light directly onor near an absorbing or non-reflective defect to ablatively remove itwhile minimizing any damage to the substrate; and

FIG. 6 is a schematic depiction of an embodiment of a repair tool forreflective photomasks which is associated with the generation andfocusing of 13 nm extreme ultraviolet light onto a reflective photomask.

DETAILED DESCRIPTION

It should be noted with the disclosure herein that the use ofprepositions, such as “on”, “over”, and “under”, are defined withrespect to a planar surface of the mask, regardless of the orientationin which the mask is actually held. A layer may be considered to be “on”another layer even if there are intervening layers.

The method of one embodiment of the invention ablatively removesspecific material using ultrashort pulses of laser light. By tuning thelaser output wavelength to match or be set within a specific offset of awavelength specific absorption of the material that is desired to beremoved, discrimination between this particular material and adjacentdesirable materials, all or part of which may be embedded in a matrix,can be achieved.

For example, and with reference to FIGS. 2A and 2B, if a first material210 possesses an atomic core level at energy E₁, and if a secondmaterial 220 possesses an atomic core level at E₂, by tuning thewavelength of ultrashort laser pulse 230 such that its energy isessentially matched to the wavelength at which material 210 has a peakenergy absorption, material 210 will be removed as ablated material210′, while material 220 will be left unaffected. Materials 210 and 220may be either elemental or compound materials. In one aspect of thisembodiment depicted in FIG. 2A, laser pulse 230 may be focused throughlens 235 at focal plane 250 just above the exposed surface of materials210 and 220, or, as depicted in FIG. 2B, laser pulse 230 may be focusedon or near the exposed surface of materials 210 and 220.

In this context, the term “ultrashort” may include pulses on the orderof a femtosecond, i.e., a quadrillionth (10⁻¹⁵) of a second, forexample.

One aspect of the above embodiment, depicted in FIGS. 3A and 3B,provides a method of repairing photomasks having two or more regions ofdiffering reflectivity at the wavelength of the incident laser lightpulses used for repair. Such a mask might include substrate 320,intended for a lithographic process, which may be highly reflective at aparticular wavelength of laser light pulses 310.

Substrate 320 may then be covered by a second region of patternedmaterial 330 that may be substantially less reflective (or absorbing) atthe chosen wavelength. The pattern of the absorbing material may be, forexample, intended for lithographic transfer to a semiconductor waferused for creating an integrated circuit. The wavelength of the laserpulses 310 used to repair defects can be tuned to maximize an absorptiondifference between the highly reflective substrate 320, and lowerreflectivity patterned absorber 330. Any defects 330′ in patternedabsorber 330 may then be selectively removed by the process of laserablation without ablating, removing or damaging substrate 320. Pulsedurations on the order of only one or a few femtoseconds may be requiredunder some conditions to effect repair. As shown in FIG. 3A, laser light310 may be focused through lens 360 at focal plane 380 just above thesurface of substrate 320 and patterned material 330, or, as illustratedin FIG. 3B, the laser may be focused on or near the surface of patternedmaterial 330, for example.

A prime candidate for next generation lithography (NGL) formanufacturing future integrated circuits on Si includes the use ofextreme ultraviolet (EUV) lithography. EUV lithography operates at awavelength of about 13 nm. Instead of a transmissive photomask, used inpresent lithographic printing technology, EUV lithography employs areflective photomask patterned with an absorber resident on top of thereflective coating.

Turning to FIG. 4, one implementation of an EUV photomask 400 includes ahigh quality multilayer substrate coated with alternating layers ofmaterials having thicknesses optimized to create an interference patternat the specific wavelength of incident laser light pulses 420. Thisinterference pattern results in enhanced reflectivity at the specificwavelength. Once the reflective layer is produced, the actual circuitpattern may be created by depositing non-reflective or absorbingmaterial 430 atop the reflective coating 410, and then patterningabsorbing material 430. Reflected EUV light 440 will transfer thepattern to a resist coated semiconductor wafer (not shown), in a processsimilar to present technology. As is the case for almost all photomaskmanufacturing, patterning of mask 400 itself is virtually never perfect.Defects in the form of excess absorber 430 occur. Since it isexceptionally expensive to create photomask 400 of this type andphotomasks, in general, and since such costs are expected to increasewith circuit complexity, it becomes critical to repair such defects byremoving any unwanted or excess absorber 430.

Conventional photomask technology involves the patterning of Cr on fusedsilica. Repairs of Cr defects may be carried out by ablating the excessCr with a laser. In this process, Cr is removed when short pulses oflaser light in the visible or UV wavelength regions are focused on thedefect. The underlying fused silica is substantially less lightabsorbing than the overlying Cr. Hence, Cr can be removed in anon-thermal process without damaging the underlying fused silica,because the ablation threshold for Cr is much lower than that fornon-absorbing glass. This contrast or absorption difference between thefused silica glass and Cr allows Cr to be removed without affecting theglass substrate.

To achieve contrast in the repair of reflective EUV photomasks, however,a new approach is required. At visible and even UV wavelengths, whereconventional photomask repair technology operates, absorber 430 andsubstrate 410 of EUV photomask 400 will both absorb the incident laserlight and ablate. Without a mechanism to provide absorption contrastbetween patterned absorber 430 and underlying reflective substrate 410,repair is impossible. Hence present technology cannot be used to repairEUV photomask 400.

To solve this problem, ablation may be carried out at the wavelengthwhere the substrate is highly reflective. The problem has not beensolved to date due to the lack of a source of pulsed laser light at 13nm (or other important EUV wavelengths), which has sufficient intensityand sufficiently short time duration to cleanly ablate solid material.With a sufficiently intense source of vacuum UV or EUV femtosecond lightpulses, the absorber can be removed. If the ablating light intensity isjudiciously chosen to have energy sufficient to remove the absorber, butlower energy than the ablation threshold of the reflective substrate,the defect can be removed with high resolution, leaving an undamagedreflective substrate behind.

Reflective photomask 500 is depicted in FIGS. 5A and 5B. In this regard,and as illustrated in FIG. 5A, when laser light pulses 510 are directedthrough lens 520 and focused at focal plane 530 just above the surfaceof substrate 550, defect 540 may be removed from substrate 550.Substrate 550 may be a multilayered reflective substrate, as discussedabove. Removed defect 540′ may be removed from substrate 550 withoutloss of reflectivity, pitting, or infliction of any damage to substrate550. Therefore, defects in the patterned absorber atop the reflectivesubstrate can be repaired by focusing an intense ultrashort durationlaser pulse above the defect, in order to ablatively remove the defect,without damaging the underlying reflective mask. Alternatively, asdepicted in FIG. 5B, the laser pulse may be focused on or near thesurface of substrate 550.

The repair process using the exemplary apparatus 600 in FIG. 6 includesseveral steps. As illustrated in FIG. 6, laser light pulses 620 fromamplified femtosecond laser 610, typically operating at 800 nm at arepetition rate of 1 kHz, is focused through an optical pathwayincluding, for example, beam control unit 670, mirror 631, focusing lens632, and window 633 into a rare gas harmonic converter cell 640containing, for example, gases such as Kr, Ar, Ne or He. The interactionof the intense laser light 620 and the gas in cell 640, along the focalplane of the laser system, is sufficient to produce harmonics of thefundamental 800 nm wavelength. Individual harmonics may be chosen byusing filter 650. Filter 650 can be a series of multilayer coatedmirrors which reject all but the desired harmonic wavelength 660. Otherselective wavelength filters may include reflective diffractiongratings, and thin metal foils, for example. Additionally, desiredharmonics may be selected by tuning of the harmonic gas pressure in cell640, or by tuning other aspects of the light generation processincluding, for example, using beam control unit 670. Beam control unit670 may also contain diagnostic equipment for evaluating the performanceof laser apparatus 600.

In one aspect of this embodiment, the selected harmonic of laser lightcomprises 13 nm (EUV) laser light, and in another aspect, the selectedharmonic of laser light may comprise 193 nm, 157 nm, or other importantharmonic wavelengths.

In particular, 13 nm light is the 61st harmonic of the 800 nm radiation.Efficiency of the harmonic generation process can be as low as 1×10⁻⁸ oras high as 1×10⁻⁵ when phase matching approaches are employed. This mayproduce as much as 100 pJ to 1 nJ of light energy at the 61 st harmonic.A beryllium window or other appropriate filter material 650 may be usedto filter out the fundamental 800 nm light component of laser light 620,permitting only the harmonic light 660 to be transmitted. In addition,phase matching approaches can be used to predominantly produce the 13 nmharmonic radiation of interest.

Once harmonic light 660 is produced, it can be focused to a diffractionlimited spot 680 by reflective microscope objective 675. Spot 680 mayhave a diameter well below 25 nm. By carefully choosing the intensity,selected absorber material on photomask 690 can be removed withoutdamaging the underlying reflecting substrate.

In terms of the particular wavelength or wavelengths chosen,lithographic printing of integrated circuitry on semiconductor wafers isgenerally carried out at specific wavelengths that are generated byefficient, high intensity excimer laser light sources, for example.Current lithographic printing utilizes the 248 nm and 193 nm emissionsfrom KrF and ArF excimer lasers, respectively. The drive to produce eversmaller feature sizes on semiconductor wafers necessitates the move toincreasingly shorter wavelengths for lithographic printing. To movebeyond 193 nm lithography, 157 nm F2 excimer lasers may be employed.Beyond this, technology employing multilayer reflective mirrors dressedwith a patterned absorber, illuminated at a wavelength of 13 nmcontinues to be developed. While 248 nm and 193 nm are deep ultravioletwavelengths (DUV), 157 nm light is in the vacuum ultraviolet (VUV),since this light is absorbed strongly in air. Laser light at 13 nm hasbeen coined “extreme UV” (EUV) light, but is also VUV light as well.

When a photomask is repaired, a number of criteria set by thelithographer must be met before the mask is considered successfullyrepaired. If a defect consisting of excess absorber must be removed,then the “repaired” region must exhibit typically better than 95% of thetransmission or reflectivity of a pristine, unrepaired region. If animproperly “repaired” defect results in poor reflectivity ortransmission, then the flux of light (248 nm, 193 nm, 157 nm, 13 nm)transmitted or reflected from this region may insufficiently expose thephotoresist on the semiconductor wafer. Resultant residual resist on thesemiconductor wafer will affect subsequent wafer processing and producesa defect on the chip. The same issues apply to line edge placement;tolerances in the degree of line edge variation after repair aredesigned to minimize the risk of transferring a defect to the chip.

In various aspects of the above embodiments of the invention, in orderto provide a more reliable mask for increased production yield and lessdowntime, a defect may be removed while maintaining at least 95% of theoriginal reflectivity of the substrate in the defect region of thesubstrate, and may maintain at least 98% of the original reflectivity ofthe substrate in the defect region. Alternatively, a reflectivity underthe removed material may be within 5% of a reflectivity of thereflective substrate in a region in which no material has been removed,or the reflectivity under the removed material may even be within 2% ofa reflectivity of the reflective substrate.

Conventional laser ablation for mask repair is typically carried outwith nanosecond pulses of laser light. Nanosecond laser ablation ischaracterized by absorption of the laser light which heats, melts andevaporates the metal. This process produces molten metal which splattersonto other parts of the mask. Thermal diffusion substantially degradesthe spatial resolution and damages the underlying substrate producingpitting and phase errors. Conversely, when light pulses in thefemtosecond (fs) range are employed, the metal is converted into aplasma on a time scale significantly shorter than the time it takes forthe excited electrons to convert their energy into heat. This“electron-phonon” coupling time is approximately 1 picosecond (ps) inmost metals. Since femtosecond pulses convert the metal directly into aplasma without substantial heat generation, diffraction limited spatialresolution (and even better) may be achieved. Picosecond light pulsesmay also be effective, but include a thermal component which degradesthe ablation process. By using femtosecond pulses, the thermal componentto the ablation process is virtually nonexistent, resulting in cleanablation and removal of defects. This feature may provide benefits inphotomask repair since the light absorbing metal can be surgicallyremoved without splatter (i.e., there is no molten metal created), andlittle creation of heat in the substrate. In the case of a reflectivephotomask, the patterned absorber sitting atop the reflective substrateabsorbs the laser pulse and is removed while the underlying reflectivesubstrate reflects the laser light and is not damaged.

The various embodiments and aspects of the invention discussed above mayinclude controlling a pulse width of the laser pulses to have a pulseduration of less than 10 picoseconds, less than 1 picosecond, less than200 femtoseconds, or less than 50 femtoseconds, to ensure accuratecontrol of the laser energy presented to the substrate and patternedmaterial, and to reduce any damage to adjacent structure which is not tobe ablated or removed.

The various embodiments of the invention discussed above may haveaspects which include a substrate which is reflective in a region oflight from 1 micron to 400 nm, 100 nm to 200 nm, 200 nm to 400 nm, 10 nmto 100 nm, or 1 nm to 10 nm to allow use in specific applications.

The various embodiments and aspects of the invention discussed above mayalso include tuning a wavelength of the laser pulses to be in a regionfrom 157 nm to 1 micron, 13 nm to 157 nm, or less than 13 nm.

The various embodiments and aspects of the invention discussed above mayalso include ablating the defect all at once, or layer by layer.

Further, the laser light may be focused directly on the defective(photomask) material to be removed, or may be focused above the surfaceof the substrate. In a recently issued IBM U.S. Pat. No. 6,333,485, theentire contents of which are incorporated herein by reference, it wasdemonstrated that focusing femtosecond pulses of laser light slightlyabove the photomask being repaired significantly reduces any residualoptical damage to the substrate. In a typical ablation process, thefocal plane of the laser light is at or below the plane of the materialbeing ablated. But for photomask repair, the intention is to remove anabsorbing material sitting atop a transmissive or reflective substrateand to minimize any possible damage to the substrate. It can bebeneficial to locate the focal plane of the ablating laser pulseslightly above the surface to ensure that the light rays are divergent,rather than convergent at the point where they interact with thesubstrate. In this manner, the light intensity is decaying as the pulsetraverses the underlying substrate, thus minimizing possible damage tothe substrate or photomask.

It will be obvious that the various embodiments of the inventiondiscussed above may be varied in many ways. Such variations are not tobe regarded as a departure from the spirit and scope of the invention,and all such modifications as would be obvious to one skilled in the artare intended to be included within the scope of the following claims.The breadth and scope of the present invention is therefore limited onlyby the scope of the appended claims and their equivalents.

1. An apparatus for repairing a defect on a reflective photomask, theapparatus comprising: a laser capable of providing femtosecond pulsewidth laser light; a harmonic conversion cell optically coupled to thelaser; a filter for blocking a fundamental wavelength and passing aselected harmonic of the laser light; an objective lens arrangementconfigured to provide the selected harmonic of the laser light onto thereflective photomask; and a control unit operatively connected to thelaser to control an ablation of the defect on the reflective photomask,wherein the selected harmonic comprises UV laser light and wherein thecontrol unit provides an absorption contrast between a patternedabsorber and an underlying reflective substrate, wherein the UV laserlight comprises 13 nm light.
 2. The apparatus of claim 1, wherein thefilter comprises a rare gas cell.
 3. The apparatus of claim 1, whereinthe control unit controls an energy level of the laser light providedonto the reflective photomask.